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We develop analytical tools and perform three-body simulations to investigate the orbital evolution and dynamical stability of binary planets within star clusters. Our analytical results show that the orbital stability of a planetary-mass binary against passing stars is mainly related to its orbital period. Critical flybys, defined as stellar encounters with energy kicks comparable to the binary binding energy, can efficiently produce a wide range of semimajor axes (a) and eccentricities (e) from a dominant population of primordially tight JuMBOs. The critical flyby criterion we derived offers an improvement over the commonly used tidal radius criterion, particularly in high-speed stellar encounters. Applying our results to the recently discovered Jupiter-Mass Binary Objects (JuMBOs) by the James Webb Space Telescope (JWST), our simulations suggest that to match the observed 9% wide binary fraction, an initial semimajor axis of a₀ 10-20 au and a density-weighted residence time of 10⁴ Myr pc^-3 are favored. These results imply that the JWST JuMBOs probably formed as tight binaries near the cluster core.
Huang et al. (Fri,) studied this question.